An Inteqrated PET-MRI System for Molecular Imaqinq Non invasive molecular imaging in small animals is an emerging field of research. Especially positron emission tomography (PET) reveals information about ongoing functional processes in living organism, such as metabolism or even gene expression. Its importance, especially in basic cancer research is tremendous. Although PET is nearly unbeatable in sensitivity (nano molar), its drawback is clearly the lack of anatomical information. Even a spatial resolution of about I mm can not provide sufficient anatomical structure. The combination of PET with another imaging device which reveals detailed morphological information would be a huge advantage and highly beneficial for molecular imaging. The modalities, providing best anatomical information are x-ray computed tomography (CT) and magnetic resonance imaging (MRI). Whereas CT requires a high radiation dose to achieve sufficient resolution, which might change biological function in the animals and potentially the entire animal model, MRI seems to be harmless. Most importantly, MRI provides a much better soft tissue contrast than CT, even without any contrast agents. The goal of this proposal is the development of an integrated PET-MRI system for simultaneous data acquisition in motecular imaging of small animals. Our latest PET detector development, in collaboration with Concorde Microsystems, Knoxville, TN, is the cornerstone for this work: a 10x10 LSO block detector (crystal size 2x2x10 mm 3) read-out by compact monolithic 3x3 APD arrays. The 10xl multiplexing minimizes the required electronics. The compact LSO-APD detectors can be arranged as an insert between RF-coil and gradient set in a 7 Tesla MR scanner, allowing simultaneous PET and MR scanning of mice. The encouraging results achieved with the LSO-APD detectors and dedicated integrated readout electronics, in combination with preliminary results from tests of different materials, such as the LSO-APD PET detectors, plastics, and electronic boards in the 7 Tesla field indicate the feasibility of this project. However, if one part of the mosaic does not allow a simultaneous data acquisition, several fall-back-solutions are worked out, to make at least a sequential PET-MR imaging possible, providing morphological and functional information with highest spatial accuracy, which could never be achieved with two separate systems. In addition, the future applications of such a combined system go far beyond the work proposed in this grant. A combined PET-MRI providing also magnetic resonance spectroscopy information or a PET data correction (attenuation or partial volume effect) based on the MR image would be very valuable and provides a lot of potential for future drug and therapy development.